Answer:
B. Without daily pumping, Manhattan's subway tunnels would be flooded
Explanation:
Cause and effect is a relationship between two or more events in which one or more events are the result of some other event. This can also be called action and reaction.
In this example the cause or action is lack of daily pumping, while effect/result/reaction of this event is flooding of Manhattan's subway tunnels.
Option A is not cause and effect because we are not told what will be the cause of glaciers covering all of Manhattan.
Option C is nor correct because there is no cause mentioned, and not even any result. It only has some information.
Option D is also incorrect because although we know a result (dying of hemlock trees), we don't know its cause.
